Plastic Deformation in a Quantum Solid: Dislocation Avalanches and Creep in Helium.

Conventional solids deform elastically for small stresses-reversibly with a linear, rate-independent relationship between stress and strain. Beyond the yield point, plastic deformation begins-irreversible, nonlinear, and time dependent. Plasticity involves the motion and multiplication of dislocations, and here we report observations of such "metallurgical" phenomena in hcp ^{4}He, a solid whose defect behavior is dominated by quantum effects. Below 0.4 K, there is a strain threshold for elastic deformation above which sudden stress drops and acoustic emissions appear, the signatures of dislocation avalanches. The dimensions of these slip events range from millimeters to microns. At higher temperatures, the avalanches are replaced by continuous creep involving dislocations, and we observe steady flow at stresses as low as 400 Pa.